1. Field of the Invention
The present invention relates generally to thermal transfer printers and particular to those pressing a thermal head against a sheet via an ink ribbon for thermal transfer.
2. Description of the Background Art
Among thermal transfer printers there is one employing an ink ribbon having yellow, magenta and cyan coloring regions and an overcoat region sequentially repeated and thermally transferred by a thermal head to a prescribed sheet to print an image in colors.
More specifically, a sheet feeding operation carrying the sheet toward the thermal head and an image printing operation sandwiching the ink ribbon between the carried sheet and the thermal head and carrying it in a direction opposite to the sheet feeding operation are performed in series repeatedly as corresponding to each coloring region of the ink ribbon. For example, if the above ink ribbon is used, the sheet will reciprocate four times.
In the sheet feeding operation the sheet is carried toward the thermal head, whereas in the image feeding operation the sheet is carried away from the thermal head. In that image feeding operation, an image will be printed on the sheet as it is carried such that it is pinched between the thermal head and a platen roller.
To reciprocate the sheet a sheet feed roller and a pinch roller are used such that the pinch roller is arranged opposite the sheet feed roller. The sheet feed roller serves as a driving roller and the pinch roller serves as a driven roller. The sheet is pinched between the sheet feed roller and the pinch roller and thus reciprocates. The driving roller or sheet feed roller will receive a motor's driving force via a prescribed gear. One such gear by way of example is a helical gear used for a driving roller's drive shaft, as described in Japanese Patent Laying-Open No. 11-160950.
Such thermal transfer printer operates, as will simply be described hereinafter. Initially, as shown in FIG. 18, in the sheet feeding operation a sheet 121 pinched between a sheet feed roller 102 and a pinch roller 104 is carried in a direction 132 as sheet feed roller 102 rotates in a direction 131. Subsequently when the sheet feeding operation completes, then as shown in FIG. 19, a thermal head 106 is pressed against sheet 121, with an ink ribbon (not shown) posed therebetween, and thus the sheet is pinched between thermal head 106 and a platen roller 108.
Then as sheet feed roller 102 rotates in a direction 134 sheet 121 pinched between thermal head 106 and platen roller 108 is carried in a direction 135, while thermal head 106 provides thermal transfer to perform the image printing operation printing an image on sheet 121. When a single image printing operation completes, the sheet feeding operation is again performed and the image printing operation is similarly performed for a subsequent color. Such an image printing operation is repeated in accordance with the ink ribbon's colors and an image is thus completely printed on the sheet.
The above described conventional thermal transfer printer, however, has the following disadvantage: to carry sheet 121, a motor (not shown) provides driving force which is in turn transmitted to sheet feed roller 102 via helical gear 114. The use of helical gear 114 contributes to limited thrust play of shaft along a longitudinal direction of a drive shaft rotating in one direction.
When the sheet feeding operation shifts to the image printing operation, however, the sheet feed roller 102 drive shaft starts to rotate in a direction opposite to that in which it rotates in the sheet feeding operation. Helical gear 114 can contribute to reduced thrust play of shaft for rotation in one direction, the gear cannot prevent thrust play of shaft when the rotation is stopped and reversed in direction.
Furthermore, when the sheet feeding operation shifts to the image printing operation, the sheet feed roller 102 drive shaft's rotation is reversed in direction 134 with sheet 121 and the ink ribbon pinched between thermal head 106 and platen roller 108. As such, with sheet 102 pinched by sheet feed roller 102 and pinch roller 104, the sheet feed roller 102 drive shaft would have thrust play of shaft in a direction 151 and sheet 121 experiences a force in a direction 141, as shown in FIG. 20.
Accordingly, sheet 121 to be carried in a direction 135 would be offset in an oblique direction 136 gradually as sheet feed roller 102 rotates, and between a sheet 121a immediately before the image printing operation and a sheet 121b after the image printing operation there will be an offset along the drive shaft (or direction 141).
As in the image printing operation sheet 121 would be offset in the thrust direction of sheet feed roller 102, an image printed on sheet 121 gradually offsets for example from a position X1 toward a position X2. In addition, this offset does not have reproducibility in degree and for each color of the ink ribbon the offset would vary in degree. This results in an offset in color providing an unclear image printed on the sheet.
If such offset in color is addressed by eliminating thrust play of shaft caused in reversing the helical gear's rotation, a thrust stopping washer or a similar, additional member is required.